In recent years, active-matrix type display devices using a current-control type self-luminous electro-optical element has been proposed. Examples of such a self-luminous electro-optical element are an organic EL element, an FED element, and an LED element. Advantages of using such a current-control type self-luminous electro-optical element are such that: (i) the number of components can be reduced because a backlight is not necessary; (ii) a degree of dependence on a viewing angle is light; and (iii) power consumption can be reduced.
The current-control type self-luminous electro-optical element means an electro-optical element that has a characteristic such that the electro-optical element itself emits light and a luminance of the light emission depends on a current.
Generally, in a current-control type self-luminous electro-optical element, a luminance is proportional to a current. Meanwhile, a relationship between the luminance and a voltage easily varies depending on, for example, a driving period or a surrounding temperature. Accordingly, it is difficult to prevent unevenness in luminance by driving, according to a voltage-control type driving method, the current-control type self-luminous electro-optical element such as an organic EL element.
It is preferable to drive, according to a current-control type driving method, the current-control type self-luminous electro-optical element that has a characteristic such that a luminance depends on a current.
Further, when a display device using a current-control type self-luminous electro-optical element is driven in an active matrix, voltage-current conversion can be carried out by a transistor constituting the active matrix. As a result, control of a current for a luminance becomes possible. Moreover, a light emission period can be freely controlled by combining the transistor with a switching element. Furthermore, it becomes possible to have a reduced power consumption or lengthening a life duration of the electro-optical element.
As a transistor constituting the conventional active matrix, a TFT (Thin Film Transistor) formed on a substrate is used. An active matrix using this TFT can achieve a light weight, a small thickness, and a high quality of the display device. Accordingly, such an active matrix is widely used for the purpose of driving an electro-optical element. As a material of the TFT, for example, amorphous silicon, low-temperature polycrystal silicon, or CG (Continuous Grain) silicon is used.
The following explains a conventional active-matrix type display device using a current-control type self-luminous electro-optical element and a driving method of the display device.
Various configurations have been proposed as an active-matrix type driving circuit using a TFT. Among the proposed configurations, the simplest configuration is a driving circuit called a 2TFT+1C (Condenser) type.
FIG. 3 is a diagram illustrating an equivalent circuit of one pixel in a 2TFT+1C type driving circuit.
As shown in FIG. 3, in a pixel 10, a second TFT 32 and an EL element 20 are provided in series in a path connecting a power supply line 4 and a ground 50. Moreover, a retention capacitor 21 and a first TFT 31 are provided in series between the power supply line 4 and a data line (Sj) 2. The first TFT 31 and the second TFT 32 are P-channel type transistors.
A gate electrode 71 of the first TFT 31 is connected to a scanning line (Gi) 3, and a gate electrode 74 of the second TFT 32 is connected to a drain electrode 73 of the first TFT 31. The second TFT 32 functions as a driving TFT for controlling an amount of current that flows into the EL element 20.
For causing a pixel 10 to emit light at a luminance in accordance with image data, a low level potential is provided to the scanning line (Gi) 3 and a potential (hereinafter, referred to as a potential Da) in accordance with the image data is provided to the data line (Sj) 2. At this time, the first TFT 31 becomes conductive and a gate electrode potential of the second TFT 32 becomes equal to the potential Da.
When the potential of the scanning line (Gi) 3 becomes a high-level potential subsequently, the first TFT 31 becomes non-conductive and the gate electrode potential of the second TFT 32 becomes fixed to the potential Da due to an influence of the retention capacitor 21.
Then, an amount of the driving current to be supplied to the EL element 20 via the second TFT 32 varies according to a gate electrode potential of the second TFT 32. The EL element 20 emits light at a luminance in accordance with the amount of the driving current supplied via the second TFT 32.
The driving current at this time is provided, in a case where the second TFT 32 operates in a saturation region, according toIOLED=1/2μ·Cox·W/L(Da−Vth)2,
where: IOLED: driving current; μ: mobility; Cox: conductance; W: channel width; L: channel length; Da: potential in accordance with image data; and Vth: threshold value.
In this way, the EL element 20 emits light at a luminance in accordance with the potential Da.
As a method of controlling the data signal line 2 or the scanning line 3, a general method is used. One example of a voltage condition or the like of a section in the circuit is disclosed in, for example, Patent Document 1.    [Patent Document 1] Japanese Patent Publication No. 3528182 (registered on Mar. 5, 2004)    [Patent Document 2] Japanese Unexamined Patent Publication No. 215296/2006 (Tokukai 2006-215296) (published on Aug. 17, 2006)    [Patent Document 3] Japanese Unexamined Patent Publication No. 47984/2006 (Tokukai 2006-47984) (published on Feb. 16, 2006)